Micromachined (MEMS) gyroscopes have become established as useful commercial items. Generally speaking, a MEMS gyroscope incorporates two high-performing MEMS devices, specifically a self-tuned resonator in the drive axis and a micro-acceleration sensor in the sensing axis. Gyroscope performance is very sensitive to such things as manufacturing variations, errors in packaging, driving, linear acceleration, and temperature, among other things. Basic principles of operation of angular-rate sensing gyroscopes are well understood and described in the prior art (e.g., Geen, J. et al., New iMEMS Angular-Rate-Sensing Gyroscope, Analog Devices, Inc., Analog Dialog 37-03 (2003), available at http://www.analog.com/library/analogDialogue/archives/37-03/gyro.html, which is hereby incorporated herein by reference in its entirety).
The principles of vibratory sensing angular rate gyroscopes with discrete masses are long-established (see, for example, Lyman, U.S. Pat. No. 2,309,853 and Lyman, U.S. Pat. No. 2,513,340, each of which is hereby incorporated herein by reference in its entirety). Generally speaking, a vibratory rate gyroscope works by oscillating a proof mass (also referred to herein as a “shuttle” or “resonator”). The oscillation is generated with a periodic force applied to a spring-mass-damper system at the resonant frequency. Operating at resonance allows the oscillation amplitude to be large relative to the force applied. When the gyroscope is rotated, Coriolis acceleration is generated on the oscillating proof mass in a direction orthogonal to both the driven oscillation and the rotation. The magnitude of Coriolis acceleration is proportional to both the velocity of the oscillating proof mass and the rotation rate. The resulting Coriolis acceleration can be measured by sensing the deflections of the proof mass. The electrical and mechanical structures used to sense such deflections of the proof mass are referred to generally as the accelerometer.
Before a MEMS gyroscope is sold, it generally must be calibrated in order to confirm the relationship between angular motion and corresponding electrical outputs produced by the gyroscope. This calibration of the transduction from angular motion to electrical output (i.e., the transduction scale factor) typically involves mounting the gyroscope to a movable mechanism and measuring the electrical outputs produced by known angular rate inputs. Calibration may be performed multiple times, for example, at the wafer level and again after final packaging, since the transduction scale factor can change during manufacturing. Calibration of the transduction scale factor is time-consuming and can be one of the most expensive elements in manufacturing cost.